When importing transactions from Account1, it should give you the option to create / reconcile transactions against existing records already in the system. If you haven't imported data from Account2 yet, then all of the transactions between the two accounts would be newly created. At this stage you would need to specify the 2nd half of the transactions as Account2.
Next, when you import the Account2 data and run through the same process, it should recognize the "matching" transactions created from the import of Account1 data, and treat them as resolved when it goes through the process of creating new transactions.
If you have already imported both data sets, then you'll need to manually clean-up the extra records. For example, if you have a transaction from
Account1 => GnuCashHinge
you need to change the target of the transfer from GnuCashHinge to Account2, so the transaction shows
Account1 => Account2
If you then close out of Account1 and view the register of Account2, you should see two transactions:
Account2 <= Account1
Account2 <= GnuCashHinge
The second transaction is a duplicate, and can be deleted. As far as I can tell, you skipped assigning the target of the transactions when importing both accounts. Thus you have duplicates of all transfer transactions, and created the Hinge account to associate the two sides of each transactions with the other. After you finish the clean-up process, the GnuCashHinge account should be empty, and can then be deleted.